Figure: a Researcher Was Interested in the Effects of Arousal and and Personality

Multiple Choice

Figure:
A researcher was interested in the effects of arousal and personality on individuals' GRE test performance.The personality variable he examined was individuals' level of introversion-extraversion.Based on questionnaire responses,each participant was classified as an "extravert" or an "introvert." The researcher manipulated arousal level by giving caffeine to participants prior to administering a practice GRE test.Participants were randomly assigned to a high-caffeine condition,a medium-caffeine condition,or the no-caffeine conditions.All participants then completed a practice version of the quantitative portion of the GRE.
Suppose the researcher observed the following means for each group:
-The natural groups independent variable in this experiment is

A) the introversion group.
B) the extraversion group.
C) level of arousal: high-caffeine,medium-caffeine,or no-caffeine.
D) participants' personality: introversion or extraversion.